In
this work, surface engineering is applied to polyimide (PI)
films to fabricate low-cost Ag/PI wireless humidity sensors with a
resonant frequency of 2.45 GHz. The sensors were obtained by in situ
metallization technique coupled with inkjet printing, where PI plays
triple roles as a flexible substrate, ion-exchange surface, and sensing
material to moisture. Moreover, the humidity sensitivity can be enhanced
by the improvement of hydrophilicity via loading with different ions
on the PI surface, which has been demonstrated by Fourier transform
infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and contact
angle measurements. The wireless humidity sensor loaded with K+ ions has the maximum sensitivity of 97.7 kHz/% RH at a low
relative humidity range of 20–65% and 359.7 kHz/% RH at a high
relative humidity of 65–90%, respectively. Accordingly, a sensing
mechanism of the fabricated humidity sensor has been discussed in
detail. On the other hand, the characteristics of the humidity sensor
such as response and recovery speed and stability are analyzed. The
mechanical performance tests show that the humidity sensor displays
excellent flexibility and good mechanical stability. A strong adhesion
between the Ag antenna and PI substrate can be found as well. The
passive wireless humidity sensor described in this work has the advantages
of having a simple structure, low cost, high sensitivity, long-term
stability, and good mechanical properties, which has potential applications
in automated industry and healthcare with real-time humidity monitoring.